Method of forming an electrocoating film, electrocoating film, and electrodeposited article

ABSTRACT

The present invention has for its object to provide a method of forming an electrocoating film having an excellent smoothness and exceptionally high dielectric properties with a drastically reduced incidence of pinhole formation, such an electrocoating film, and such an electrodeposited article. A method of forming an electrocoating film comprising coating a work with an electrocoating composition curable by heating and irradiation with an activation energy beam in which an electrodepositing step, an aqueous cleaning step, a pre-baking step, an activation energy beam irradiation step, and a post-baking step are serially carried out in the order mentioned.

TECHNICAL FIELD

The present invention relates to a method of forming an electrocoatingfilm, an electrocoating film, and an electrodeposited article.

BACKGROUND TECHNOLOGY

Recently attempts have been made to form an insulating coating film froma coating on a metallic surface.

Regarding the substrate of a multi-layer printed wiring board, forinstance, a metallic substrate formed with an epoxy resin insulatinglayer on which an electric circuit is formed by electroless plating forimproved heat dissipation, called a metal core PWB, has so far beendeveloped (e.g. Itoh, Kinji: Introduction to Manufacture of PrintedWiring Boards for Certification of Printed Wiring Board Engineers, 1stEdition, Jul. 6, 2001, pp. 67-69).

However, the recent trend toward higher-integration and higher-densitysubstrate boards presents with the problem of leaks between the metalsubstrate and the circuit due to the minute pinholes existing in theinsulating layer and, therefore, this insulating layer is required tohave high dielectric properties.

Electrocoating compositions have excellent throwing powers and yieldcomparatively even coats regardless of the shape of work as well assufficient dielectric properties so that attempts have been made to forminsulating layers using electrocoating compositions. Although suchattempts are rewarded with several advantages of electrocoating, thiselectrocoating film is usually not so flat and smooth as desired andtends to develop pinholes owing to evolution of hydrogen gas in theelectodepositing process, thus making it difficult to implement ahigh-level dielectric property using an electrocoating composition.

SUMMARY OF THE PRESENT INVENTION

The present invention has for its object to provide a method of formingan electrocoating film having an excellent smoothness and exceptionallyhigh dielectric properties with a drastically reduced incidence ofpinhole formation, such an electrocoating film, and such anelectrodeposited article.

The present invention is first directed to a method of forming anelectrocoating film comprising coating a work with an electrocoatingcomposition curable by heating and irradiation with an activation energybeam

in which an electrodepositing step, an aqueous cleaning step, apre-baking step, an activation energy beam irradiation step, and apost-baking step are serially carried out in the order mentioned.

The activation energy beam irradiation step may be carried out directlyfollowing said pre-baking step without cooling the work.

The heating in the above post-baking step may be continuous from thepre-baking step.

Preferably, the electrocoating composition comprises a resin compositioncontaining sulfonium and propargyl groups.

Preferably, the electrocoating composition is a cationic electrocoatingcomposition.

The present invention is further directed to an electrocoating film

which is formed by the above method of forming an electrocoating film.

In addition, the present invention is directed to an electrodepositedarticle having the electrocoating film.

The present invention is further directed to a method of forming amultilayer film

in which the above electrocoating film is further coated with anovercoat.

The present invention is further directed to a multilayer film

which is formed by the method of forming a multilayer film.

Furthermore, the present invention is directed to an article having themultilayer film.

DISCLOSURE OF INVENTION

The method of forming an electrocoating film according to the presentinvention is a method of forming an electrocoating film which comprisescoating a work with an electrocoating composition curable by heating andirradiation with an activation energy beam, and comprises anelectrodepositing step, an aqueous cleaning step, a pre-baking step, anactivation energy beam irradiation step, and a post-baking step.

The electrocoating composition mentioned above contains a bindercomponent curable by heating and irradiation with an activation energybeam in addition to the ionic groups necessary for electrodeposition.The binder component may be a component having a functional groupcurable by heating and irradiation with an activation energy beam or onehaving both a heat-curable functional group and an activation energybeam-curable functional group, and whichever of these can be employed.

The curability by irradiation with an activation energy beam, asreferred to above, includes not only the mode in which a curing reactionis directly caused by an activation energy beam itself but also the modein which a curing reaction is caused by the active species generated byan activation energy beam. The activation energy beam-curable functionalgroup may include an unsaturated bond, such as a double bond or a triplebond, a combination of the above unsaturated bond and a thiol group, anepoxy group, a maleimide group, an oxetane group, an alkoxysilyl groupand so on, but in consideration of the stability of coexistence withother functional groups, either an unsaturated bond or a combination ofthe above unsaturated bond and a thiol group is preferred. Moreover,said unsaturated bond is not only curable by irradiation with anactivation energy beam but may be converted to a heat-curable functionalgroup by formulating a heat-sensitive radical initiator, such as andialkyl peroxide, peroxycarboxylic acid, peroxycarbonate, peroxy-ester,hydroperoxide, ketone peroxide, azodinitrile or benzopinacol silylether, in the coating.

Further, as the heat-curable functional group, those functional groupswhich are well known in the coating art can be utilized. The functionalgroup mentioned above is not particularly restricted unless it directlyreacts with said ionic group necessary for electrodeposition or saidactivation energy beam-curable functional group or interferes with theelectodepositing process or the curing by an activation energy beam.Specifically, the preferred one is hydroxyl group. As regards thisfunctional group capable of curing on exposure to heat, a curing agentserving as a partner in the curing reaction is usually incorporated inthe coating composition. In the case where the functional group capableof curing on exposure to heat is hydroxyl, the curing agent which can beused may be any of those well known in the art, such as an optionallyblocked polyisocyanate or melamine resin, for instance.

For example, Japanese Kokai Publication Hei-05-263026 discloses aUV-curable cationic elecrocoating composition comprising 10 to 70 partsby weight of a polyfunctional acrylate having three or more acryloylgroups within the molecule and 30 to 90 parts by weight of a resinsuitable for cationic electrodeposition and having an average molecularweight of 2000 to 30000 as active components. Such electrocoatingcompositions containing a binder component having an unsaturated bond asan activation energy beam-curable functional group are known and,therefore, by incorporating a heat-sensitive radical initiator in such aknown coating, there can be provided an electrocoating composition foruse in the method of forming an electrocoating film according to thepresent invention. Furthermore, it is not difficult for those skilled inthe art to provide an electrocoating composition for use in the methodof forming an electrocoating film by introducing a heat-curablefunctional group into a binder component having said unsaturated bond asan activation energy beam-curable functional group and selecting acuring agent compatible therewith.

Furthermore, an aqueous dispersion comprising a cathionicgroup-containing polyurethane (meth)acrylate having an ethylenicallyunsaturated terminal (meth)acryloyl double bond and a reactive diluenthaving at least two ethylenically unsaturated (meth)acryloyl doublebonds as a binder component and a light-sensitive radical initiatorand/or a heat-sensitive radical initiator, which is disclosed inJapanese Kohyo Publication 2002-531676, can be used as theelectrocoating composition for use in the method of forming anelectrocoating film according to the present invention. Here, the(meth)acryloyl double bond in said aqueous dispersion has a brominevalue equal to 20 to 150 g bromine/100 g solids and the terminal(meth)acryloyl double bond of ethylenic unsaturation from saidpolyurethane (meth)acrylate is bound to a cationic group-containingpolyurethane prepolymer through a urethane, urea, amide or ester group.

Furthermore, a cationic electrocoating composition containing sulfoniumand propargyl groups is disclosed in WO 98/03595. This electrocoatingcomposition, too, can be used in the method of forming an electrocoatingfilm according to the present invention. The sulfonium group content ofthis electrocoating composition per 100 g of resin solids is 5 to 400millimoles and the propargyl group content on the same basis is 10 to495 millimoles, the total content of sulfonium and propargyl groupsbeing not greater than 500 millimoles. Furthermore, a componentcontaining a double bond as unsaturation in addition to propargyl groupmay also be used in combination. Incidentally, it is known that saidcationic electrocoating composition containing sulfonium and propargylgroups undergoes curing when heated even in the absence of theheat-sensitive radical initiator. Moreover, because it contains thepropargyl group being an unsaturated bond, the coating can be cured byirradiation with an activation energy beam.

The electrocoating composition for use in the method of forming anelectrocoating film according to the present invention may be whicheverof an anionic electrocoating composition and a cationic electrocoatingcomposition but is preferably a cationic electrocoating composition inview of the release of ions from the work.

From the standpoint of dielectric property of the formed electrocoatingfilm, the electrocoating composition for use in the method of forming anelectrocoating film according to the present invention is preferablysaid cationic electrocoating composition containing sulfonium andpropargyl groups. As such cationic electrocoating composition, there canbe mentioned Insuleed Series (electrolytically active electrocoatings,manufactured by Nippon Paint Co.)

Where necessary, the electrocoating composition for use in the method offorming an electrocoating film according to the present invention maycontain a light-sensitive radical initiator. As the light-sensitiveradical initiator which can be comprised in said electrocoatingcomposition, various compounds which are well known to those skilled inthe art can be employed. Among such substances are benzoin compoundssuch as benzoin, benzoin isopropyl ether, benzoin isobutyl ether, and soon; benzophenone compounds such as benzophenone,4,4′-bis(dimethylamino)benzophenone (Michler's ketone), and so on;xanthone compounds such as xanthone, thioxanthone, and so on;acetophenone compounds such as 2-phenyl-2-hydroxyacetophenone,α,α-dichloro-4-phenoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone,2,2-diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-2-methylpropiophenone, 2,2-dimethoxy-2-phenylacetophenone(benzyl dimethyl ketal), and so on; ethyl 4-dimethylaminobenzoate,4,4′-diazidostilbene-2,2′-disulfonic acid,1-phenyl-1,2-propanedione-2-(o-ethoxycarbonyl)oxime, and so forth. Theselight-sensitive radical initiators are usually formulated in aproportion of 0.1 to 10% by weight based on the resin solids of thecoating composition.

The electrocoating composition for use in the method of forming anelectrocoating film according to the present invention may contain thosepigments and additives which are conventionally formulated inelectrocoating compositions.

The work for use in the method of forming an electrocoating filmaccording to the present invention is not particularly restrictedprovided that the area to be coated is electrically conductive. There isno restriction on the shape of the work.

The electrodepositing step in the method of forming an electrocoatingfilm according to the present invention comprises electrodepositing anelectrically conductive work with the above-described electrocoatingcomposition to give an uncured electrodeposited coat. The relevantelectrodepositing conditions such as electrodepositing bath temperature,electrodepositing voltage and current time are established according tothe particular electrocoating composition to be used but are preferablyso set as to yield a dry film thickness of 5 to 30 μm.

The aqueous cleaning step in the method of forming an electrocoatingfilm according to the present invention, which is carried out oncompletion of said electrodepositing step, comprises washing off thesuperfluous electrocoating composition remaining on the work andelectrodeposited coat. The solvent for use in this aqueous cleaning stepis preferably deionized water from the standpoint of dielectric propertyof the resulting electrocoating film but there may optionally be used atwo-stage method which comprises prewashing with a mixture solvent ofdeionized water and a water-soluble carboxylic acid compound, such asacetic acid, lactic acid or the like, and postwashing with deionizedwater. The residual electrocoating composition can be thereby removedmore effectively.

The mode of the above aqueous cleaning is not particularly restrictedbut any of those techniques which are well known in the art, such asdipping and spraying, can be employed. The cleaning time is notparticularly restricted but may, for example, be 30 seconds to 2minutes.

The pre-baking step which is carried out on completion of said aqueouscleaning step in the method of forming an electrocoating film accordingto the present invention is not intended to cure the whole of saidelectrodeposited coat but intended to melt and let flow the uncuredelectrodeposited coat once formed to thereby eliminate film defects suchas pinholes for improved evenness and surface smoothness of the coat.Therefore, even in the case where the heating is further continued, thisstep is regarded as having been completed at the time-point where theelectrodeposited coat has been molten and let flow. The melting andletting-flow of the uncured electrodeposited coat in the above manner isadvantageous in that an improvement in the smoothness of the coat aswell as elimination of pinholes in the coat can be accomplished toinsure a high degree of dielectric property.

Furthermore, improving the smoothness of the coat in this way enablesthe use of the obtained film in an electric circuit utilizinghigh-frequency signal. This is because a reduced coarseness of the coatleads to a reduction in the disturbance of the magnetic field by acurrent so that the product can be made adaptable to high-frequencycurrent applications which are apt to be influenced by a magnetic field.Moreover, the improved surface smoothness can reduces the risk fordisconnection of a conductor locating on an electrodeposited article andenables fine-lined conversion of the electrodeposited article on itspatterning as well.

The heating temperature for use in the above pre-baking step should, ofnecessity, be not lower than the temperature causing theelectrodeposited coat to melt and it can be judiciously selectedaccording to the kind of electrocoating composition used but isgenerally within the range of 60 to 130° C. If it is below 60° C., theflow of the uncured electrodeposited coat on melting tends to beinsufficient so that pinholes may remain after this step and theevenness or the surface smoothness of the coat may be insufficient. Onthe other hand, if the temperature exceeds 130° C., the uncuredelectrodeposited coat may begin to cure before the coat has flownsufficiently, with the consequence that, after this step, pinholes mayremain, the evenness of the coat be inadequate, or the surfacesmoothness of the coat be insufficient. The preferred range is 70 to110° C. The heating time in said pre-baking step is not particularlyrestricted but from coat meltability and industrial points of view, itmay for example be 2 to 30 minutes.

The activation energy beam irradiation step in the method of forming anelectrocoating film according to the present invention, which precedesthe post-baking step, is intended to fix the coat surface which has beenimproved in evenness and smoothness by said pre-baking step. By carryingout this step, the reflow of the electrodeposited coat is inhibited sothat even when the electrodeposited coat is further heated and caused tocure thoroughly, the ultimate electrocoating film can be obtained withthe high surface smoothness achieved in this step being successfullyretained.

The activation energy beam which can be used includes ultraviolet light,X-rays, an electron beam, near-infrared light and visible light. Theactivation energy beam in the context of the present invention does notinclude heat-generating energy beams such as infrared light,high-frequency waves and microwaves. However, although near-infraredlight is a heat-generating energy beam, it is included in saidactivation energy beam because there exist light-sensitive radicalinitiators showing initiating functions in this wavelength region.

For irradiation with ultraviolet light, there can be used a variety oflight sources, such as a mercury arc lamp, a xenon arc lamp, afluorescent lamp, a carbon arc lamp, a tungsten-halogen copier lamp, andso forth. On the other hand, as emission sources of an electron beam,there can be used electron beam generators such as Cockcroft type,Cockcroft-Walton type, van de Graaff type, resonance transformer type,transformer type, insulating core transformer type, Dynamitron type,linear filament type, high-frequency type, and other devices. It shouldbe understood that when an electron beam is used, it is not alwaysnecessary to use a light-sensitive radical initiator.

The conditions of said irradiation with an activation energy beam inthis step vary with the amount of unsaturation in the resin and themolecular weight of the resin in the electrocoating composition used buttaking ultraviolet light as an example of said activation energy beam,its wavelength range may be 200 to 500 nm and the integrating radiationdosage may for example be 100 to 10000 mJ/cm² when the coating usedcontains a light-sensitive radical initiator or 1000 to 20000 mJ/cm²when the coating does not contain a light-sensitive radical initiator.When the integrating radiation dosage is insufficient, fixing of theelectrodeposited coat surface is inadequate so that the ultimateelectrocoating film tends to be deficient in smoothness. An excessiveintegrating radiation dosage would not cause any serious trouble butlead to a waste of energy. On the other hand, when an electron beam isused, it is advantageous to carry out the irradiation using an electronbeam generator with an output energy of 50 to 500 keV for apredetermined time.

It is also recommendable to adjust the distance between theelectrodeposited coat and the light source according to the shape of thework so as to provide for a uniform irradiation with the activationenergy beam and bring said integrating radiation dosage and energy intoa predetermined range.

The above activation energy beam irradiation step may be carried outdirectly following said pre-baking step without cooling the work betweenthe steps.

The post-baking step in the method of forming an electrocoating filmaccording to the present invention is intended to thermally cure theelectrodeposited coat having a fixed surface following the activationenergy beam irradiation step. By carrying out this step, the entirety ofthe electrodeposited coat inclusive of its interior can be caused tocure thoroughly. Although the electrodeposited coat undergoes remeltingto flow in this step, the coat surface which has been hardened in thepreceding step does not melt so that an ultimately cured electrocoatingfilm can be produced with the high surface smoothness obtained in thepreceding step being fully retained.

The heating conditions for use in this post-baking step are notparticularly restricted provided that the electrocoating film having afixed surface obtained after said pre-baking step and subsequentactivation energy beam irradiation step can be thoroughly cured down toits interior. The above heating conditions can be appropriatelyestablished according to the kind of electrocoating composition usedand, for example, the heating temperature may be 130 to 260° C. Theheating time in this post-baking step is not particularly restricted,either, and may for example be 10 to 30 minutes.

The heating in this post-baking step may be a continuation of theheating in said pre-baking step.

The method of forming an electrocoating film according to the presentinvention is applicable to electrically conductive substrates and can beused to form electrocoating films on various metallic materials such ascopper, iron, galvanized steel sheet, aluminum and so forth.

The electrocoating film of the present invention is formed by theabove-described method of forming an electrocoating film, and havingsuch a film, the electrodeposited article of the present inventionfeatures very satisfactory dielectric properties and surface smoothness.

The method of forming a multilayer film according to the presentinvention may comprise applying an overcoat in superimposition on thecured electrocoating film described above. The overcoat is intended toprotect and impart an attractive appearance to said electrocoating filmor adding new functions to the multilayer film. The overcoat mentionedjust above is not particularly restricted but includes, for example,those materials which can be caused to undergo a curing reaction byheating and/or irradiation with an activation energy beam. Amongspecific examples are those binders, among the binder componentsmentioned in the foregoing description of the electrocoating compositionof the present invention, which can be cured by heating and/orirradiation with an activation energy beam. In the case of a non-aqueousone, the binder need not have an ionic group. As the skeleton of thebinder component, there can be used an acrylic resin, polyester resin,epoxy resin, urethane resin, or the like and where flexibility isfurther required, these resins may have been modified by introducing abutadiene skeleton, siloxane skeleton or long-chain aliphatic skeleton.

In the case where the overcoat is heat-curable but cannot be cured inthe presence of said binder component alone, a curing agent suited tothe kind of said reactive functional group can be used as an auxiliarybinder component. The curing agent mentioned just above includes aminoresins and optionally blocked polyisocyanates.

In addition to said binder component or components, the above overcoatmay contain a pigment, a resin particle, and various additives. Forimparting electrical conductivity as a new function to the multilayerfilm, the above overcoat may be supplemented with a metal particle,carbon, metal oxide, and/or the like. For imparting high dielectricproperties, glass fiber, ceramics, and so on may be formulated.

The coating method is not particularly restricted but includes thetechniques well known to those skilled in the art, such as bar coating,die coating, spray coating, rotary atomizer coating, spin coating, andso forth. By curing the resulting film by heating and/or irradiationwith an activation energy beam, an overcoat can be constructed. Theheating conditions and the conditions of activation energy beamirradiation can be judiciously established according to the kind ofovercoat used. The cured thickness of the above overcoat is notparticularly restricted but may, for example, be 10 to 100 μm.

The application of the overcoat may be carried out in a few dividedcycles.

The multilayer film of the present invention is the film obtained by theabove-described method of forming a multilayer film, and the articleaccording to this invention has said multilayer film and, therefore,features a high degree of smoothness.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

The following examples are intended to illustrate the present inventionin further detail without defining the scope of the present invention.In the examples, “part(s)” means “part(s) by weight” unless otherwisespecified.

EXAMPLE 1

Using Insuleed 1004 (product of Nippon Paint Co., an electrolyticallyactive electrocoating, melting temperature 90° C., curing temperature180° C.), a copper substrate (10 cm×10 cm×700 μm thick) waselectrodeposited at an electrodepositing voltage of 200 V for 1 minutein a dry film thickness of 20 μm to give an uncured electrodepositedcoat. The temperature of the electrodepositing bath was 30° C. Afterelectrodepositing, the substrate was cleaned by dipping it in deionizedwater for 1 minute to remove the superfluous electrocoating compositionon the substrate and electrodeposited coat.

Then, the substrate carrying this uncured electrodeposited coat waspre-baked in a heating oven at a set temperature of 90° C. for 10minutes. After completion of this pre-baking, the substrate was takenout from the oven and the electrodeposited coat on the substrate surfacewas visually examined. It was found that the coat had flown sufficientlyto become smooth.

Then, using a high-pressure mercury-vapor lamp (peak wavelength 365 nm,irradiation intensity 50 mJ/(cm²·s)), the substrate was irradiated withultraviolet light at an integrating radiation dosage of 10000 mJ/cm².

Thereafter, the substrate was post-baked in a heating oven at a settemperature of 180° C. for 20 minutes to give an electrocoating film.

EXAMPLE 2

A substrate having an uncured electrodeposited coat as obtained in thesame manner as in Example 1 was used as the work. Meanwhile, a devicecapable of effecting both heating and UV irradiation was set to aheating temperature of 90° C. and, for UV irradiation, to a peakwavelength of 365 nm, an irradiation intensity of 50 mJ/(cm²·s) and anintegrating radiation dosage of 10000 mJ/cm². Using the above device,heating alone was carried out for 9 minutes. Then, with the heatingbeing further continued under the same conditions as above for 1 minute,the work was irradiated with UV light for 1 minute.

Thereafter, the work was caused to cure by the post-baking in a heatingoven set to 180° C. for 20 minutes to give an electrocoating film.

EXAMPLE 3

Using CCR-232GF (product of Asahi Chemical Research Co., an epoxy resinovercoat), the electrocoating film obtained in Example 2 wasspray-coated in a cured thickness of 25 μm. Then the work was cured byheating at 150° C. for 60 minutes to give a multilayer film.

EXAMPLE 4

Using a coating material prepared by adding 1 part of Irgacure 651(benzyl dimethyl ketal, product of Chiba-Geigy, a light-sensitiveradical initiator) to 100 parts resin solids of Insuleed 1004 in lieu ofInsuleed 1004, the procedure of Example 1 was otherwise faithfullyfollowed to give an uncured electrodeposited coat which had flownsufficiently to present with a smooth surface.

Then, except that the integrating radiation dosage of a high-pressuremercury vapor lamp was set to 200 mJ/cm², the work was post-baked in thesame manner as in Example 1 to give an electrocoating film.

COMPARATIVE EXAMPLE 1

Omitting the pre-baking and UV irradiation steps, the procedure ofExample 1 was otherwise faithfully followed to give an electrocoatingfilm.

COMPARATIVE EXAMPLE 2

Omitting the pre-baking step, the procedure of Example 1 was otherwisefaithfully followed to give an electrocoating film.

COMPARATIVE EXAMPLE 3

Omitting the UV irradiation step, the procedure of Example 1 wasotherwise faithfully followed to give an electrocoating film.

<Evaluation Tests>

Smoothness

Using SJ-201 (the surface roughness tester manufactured by MitsutoyoCo.), the surface roughness Ra values of the electrocoating filmsobtained in Examples 1, 2 and 4 and in Comparative Examples 1-3 and thecorresponding value of the multilayer film obtained in Example 3 wererespectively measured. As measuring conditions, a cutoff point of 0.8 mmwas used. The results are presented in Table 1.

Dielectric Breakdown Voltage

The dielectric breakdown voltage values of the electrocoating filmsobtained in Examples 1, 2 and 4 and Comparative Examples 1 to 3 and thecorresponding value of the multilayer film obtained in Example 3 wererespectively measured with Auto Tester Model 8525 (the breakdown voltagetester manufactured by Tsuruga Electric Co.). The results are presentedin Table 1. TABLE 1 Example Comparative Example 1 2 3 4 1 2 3 Smoothness(μm) 0.15 0.22 0.20 0.16 0.35 0.67 0.35 Dielectric property (KV) 4.6 4.97.5 4.4 2.5 0.8 2.7

It will be apparent from Table 1 that the electrocoating films (Examples1, 2 and 4) obtained by the method of forming an electrocoating filmaccording to the present invention and the multilayer film (Example 3)obtained by the method of forming a multilayer film according to thepresent invention are outstanding in smoothness and dielectricproperties. However, the film obtained by omitting the pre-baking step(Comparative Example 2), the film obtained by omitting the pre-bakingand activation energy beam irradiation steps (Comparative Example 1) andthe film obtained by omitting the activation energy beam irradiationsteps (Comparative Example 3) were inferior in surface smoothness anddielectric properties.

INDUSTRIAL APPLICABILITY

The method of forming an electrocoating film according to the presentinvention comprises a pre-baking step and an activation energy beamirradiation step which are carried out in that order prior to apost-baking step, with the result that the resulting electrocoating filmis outstanding in surface smoothness and dielectric properties.

Moreover, the method of forming a multilayer film according to thepresent invention comprises applying an overcoat in superimposition onthe electrocoating film obtained by said method of forming anelectrocoating film and the resulting multilayer film is alsooutstanding in surface smoothness and dielectric properties.

The underlying principle of these methods is that the surface roughnessof the uncured coat formed by electrodepositing and the pinholesexisting in the coat can be reduced or eliminated by the pre-baking stepwhich causes the uncured electrodeposited coat to melt and flow, thusproviding for evenness and smoothness, the film surface is then fixed topreserve said evenness and smoothness by irradiation with an activationenergy beam and, thereafter, the entirety of the coat is cured bypost-baking.

The method of forming an electrocoating film according to the presentinvention and the method of forming a multilayer film according to thepresent invention provide an electrocoating film and a multilayer film,both having every satisfactory surface smoothness and dielectricproperties and, thus, finding application in various kinds of electronicand electric equipment.

1. A method of forming an electrocoating film comprising coating a workwith an electrocoating composition curable by heating and irradiationwith an activation energy beam in which an electrodepositing step, anaqueous cleaning step, a pre-baking step, an activation energy beamirradiation step, and a post-baking step are serially carried out in theorder mentioned.
 2. The method of forming an electrocoating filmaccording to claim 1, wherein said activation energy beam irradiationstep is carried out directly following said pre-baking step withoutcooling the work.
 3. The method of forming an electrocoating filmaccording to claim 1, wherein the heating in said post-baking step iscontinuous from said pre-baking step.
 4. The method of forming anelectrocoating film according to claim 1, wherein said electrocoatingcomposition comprises a resin composition containing sulfonium andpropargyl groups.
 5. The method of forming an electrocoating filmaccording to claim 1, wherein said electrocoating composition is acationic electrocoating composition.
 6. An electrocoating film which isformed by the method of forming an electrocoating film according toclaim
 1. 7. An electrodeposited article having the electrocoating filmaccording to claim
 6. 8. A method of forming a multilayer film in whichthe electrocoating film according to claim 6 is further coated with anovercoat.
 9. A multilayer film which is formed by the method of forminga multilayer film according to claim
 8. 10. An article having themultilayer film according to claim
 9. 11. The method of forming anelectrocoating film according to claim 2, wherein the heating in saidpost-baking step is continuous from said pre-baking step.
 12. The methodof forming an electrocoating film according to claim 2, wherein saidelectrocoating composition comprises a resin composition containingsulfonium and propargyl groups.
 13. The method of forming anelectrocoating film according to claim 3, wherein said electrocoatingcomposition comprises a resin composition containing sulfonium andpropargyl groups.
 14. The method of forming an electrocoating filmaccording to claims 2, wherein said electrocoating composition is acationic electrocoating composition.
 15. The method of forming anelectrocoating film according to claim 3, wherein said electrocoatingcomposition is a cationic electrocoating composition.
 16. The method offorming an electrocoating film according to claim 4, wherein saidelectrocoating composition is a cationic electrocoating composition. 17.An electrocoating film which is formed by the method of forming anelectrocoating film according to claim
 2. 18. An electrocoating filmwhich is formed by the method of forming an electrocoating filmaccording to claim
 3. 19. An electrocoating film which is formed by themethod of forming an electrocoating film according to claim
 4. 20. Anelectrocoating film which is formed by the method of forming anelectrocoating film according to claim 5.